in recent years there has been growing interest in the use of polymers for electronic applications. One particular area of importance is organic photovoltaics (OPV). Polymers have found use in OPVs as they allow devices to be manufactured by solution-processing techniques such as spin casting, dip coating or ink jet printing. Solution processing can be carried out cheaper and on a larger scale compared to the evaporative techniques used to make inorganic thin film devices. Currently, polymer based devices are achieving efficiencies up to 4-5% (see for example K. M. Coakley and M. D. McGehee, Chem. Mater. 2004, 16, 4533-4542). This is appreciably lower than the efficiencies attainable by inorganic devices, which are typically up to 25%.
The class of polymers currently achieving the highest efficiencies in OPV devices are poly(3-alkyl-thiophenes) (P3AT). The most commonly used example is poly(3-hexyl-thiophene) (P3HT), due to its broad availability and good absorption characteristics. P3HT absorbs strongly over the range from 480 to 650 nm, with a peak maximum absorption at 560 nm. This means a significant portion of the light emitted by the sun is not being absorbed.
In order to improve the efficiency of OPV devices, polymers are required that absorb more light from the longer wavelength region (650 to 800 nm) of the solar spectra. For this purpose, polymers are desired which have a low band gap, preferably less than 1.9 eV, whereas for example P3HT has a band gap of ˜2.0 eV.
Besides OPV devices, organic materials have also shown promise as the active layer in organic based thin film transistors and organic field effect transistors (TFT, OFET) (see H. E. Katz, Z. Bao and S. L. Gilat, Acc. Chem. Res., 2001, 34, 5, 359). Such devices have potential applications in smart cards, security tags and the switching element in flat panel displays. Organic materials are envisaged to have substantial cost advantages over their silicon analogues if they can be deposited from solution, as this enables a fast, large-area fabrication route.
The performance of the device is principally based upon the charge carrier mobility of the semi-conducting material and the current on/off ratio, so the ideal semiconductor should have a low conductivity in the off state, combined with a high charge carrier mobility (>1×10−3 cm2 V−1 s−1). In addition, it is important that the semi-conducting material is relatively stable to oxidation i.e. it has a high ionisation potential, as oxidation leads to reduced device performance.
Regioregular head-to-tail P3HT has been reported with charge carrier mobility between 1×10−5 and 4.5×10−2 cm2V−1 s−1, but with a rather low current on/off ratio between 10 and 103 [see Z. Bao et al., Appl. Pys. Lett., 1996, 69, 4108]. This low on/off current is due in part to the low ionisation potential of the polymer, which can lead to oxygen doping of the polymer under ambient conditions, and a subsequent high off current [see H. Sirringhaus et al., Adv. Solid State Phys., 1999, 39, 101].
A high regioregularity leads to improved packing and optimised microstructure, leading to improved charge carrier mobility [see H. Sirringhaus et al., Science, 1998, 280, 1741-1744; H. Sirringhaus et al., Nature, 1999, 401, 685-688; and H. Sirringhaus, et al., Synthetic Metals, 2000, 111-112, 129-132]. In general, P3AT show improved solubility and are able to be solution processed to fabricate large area films. However, P3AT have relatively low ionisation potentials and are susceptible to doping in air.
It is an aim of the present invention to provide new materials for use as semiconductors or charge transport materials, especially for use in OPV and OFET devices, which are easy to synthesize, have high charge mobility, good processibility and oxidative stability. Another aim of the invention is to provide new semiconductor and charge transport components, and new and improved electrooptical, electronic and luminescent devices comprising these components. Other aims of the invention are immediately evident to those skilled in the art from the following description.
The inventors of the present invention have found that these aims can be achieved by providing regioregular polyselenophenes as claimed in the present invention. In particular, it was found that regioregular poly(3-alkyl)selenophene (P3AS) and other polymers containing selenophene exclusively in the backbone have bandgaps lower than 1.9 eV, whilst maintaining the desirable properties of high hole carrier transport, solution processablilty and high optical absorption coefficients.
Regiorandom poly(3-alkyl)selenophene has previously been synthesised by the electrochemical polymerisation of a 3-alkyl selenophene precursor (C. Mahatsekake et al, Phosphorus, Sulfur and Silicon, 1990, 47, 35-41) or by a oxidative chemical route utilising FeCl3 (Y. Katsumi et al Japanese Journal Appl. Physics. Part 2, 1989, 28, L138-L140; C. G. Andrieu et al, Sulfur Letters, 1996, 19, 261-266). Both preparations afforded material of low regioregularity with respect to the positioning of the alkyl side chains. The regiorandom poly(3-alkyl)selenophene reported by Katsumi and co-workers (Y. Katsumi et al Japanese Journal Appl. Physics. Part 2, 1989, 28, L138-L140) was reported to have a bandgap larger than regiorandom poly(3-alkyl)thiophene (2.4 eV vs 2.2 eV). This was rationalized by the increased steric interactions between the alkyl sidechains and the larger ionic radius of selenium in comparison to sulfur, which decreases the planarity, and thus the bandgap, of the polymer backbone. The charge carrier mobility of these materials in field effect transistors or the performance in organic photovoltaic devices has not been previously reported.
EP-A-1 439 590 discloses mono-, oligo- and poly-bis(thienyl) arylenes, but does not disclose the polymers of the present invention. S. Tierney, M. Heeney and 1. McCulloch, Synth Met., 148(2), 195-198, (2005) discloses poly-bis(3-octyl-thiophen-2-yl) selenophene, but does not disclose polymers of the present invention.